The present invention is directed to a fixation device for use in testing eyes, particularly children's eyes.
The use of accommodative targets is of unquestioned value in the evaluation of strabismus, a misalignment of the visual axes of the eyes. An accommodative target is a target which requires the patient to focus his or her eyes. A line of letters on a chart are accommodative targets, whereas light is a non-accommodative target.
In a co-operative adult, reading letters from a standard eye chart allows the opthalmologist to evaluate the alignment of the eyes. Children, however, are not as co-operative and either will not or cannot read letters from a chart. As a result, there are numerous devices that are intended to maintain a child's fixation at a distant point so that the alignment of the child's eyes can be examined.
One such device employs a film projector which projects a real image on the wall at the far end of the examination room. Appropriate images, such as animated cartoon characters, are used to interest the child and maintain fixation on the image.
There are, however, several technical problems with this device. First, the noise of the projector is often a distraction for the child. Second, the mechanical nature of the device makes it failure prone. More significantly, children generally view film in a passive, often disinterested manner, and as they become familiar with the cartoons that are inserted into the projectors their attention span is limited even on subsequent visits.
Other attempts to provide fixation devices for children have also exhibited limitations. One device consists of lights which are positioned so that when they are illuminated sequentially, they draw the patient's fixation to a central point. For example, lights positioned along the lines of an X can be illuminated to draw fixation toward the center of the X. Concentric circles can be illuminated in succession to draw attention to the center. Such devices, however, require co-operation from the patient and use a non-accommodative target, light. The non-accommodative target can seriously alter the strabismus evaluation.
Other fixation devices employ mechanical figures, e.g., dogs, clowns. When activated, the figures move, often make noise and provide, at least momentarily, an ideal fixation target. Unfortunately, as children become bored with the repetitive movements and sounds of the figures the fixation can be lost. To counteract this problem, a device employing numerous mechanical figures has been constructed. Each figure has its own source of illumination and when the child tires of one figure, the examiner simply turns it off and turns on another. Installation of this device, with all its interconnections, is a minor problem. More importantly, the examiner cannot devote his or her entire attention to evaluating the patient since the targets must be continually switched off and on.